Footnotes

• Contributors MC had the idea for the study; IL, SM and MR undertook all the measurements; AMcF was responsible for the statistical analyses; IL and CI wrote the first draft; all authors contributed to the final draft.

• Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests None declared.

• Provenance and peer review Not commissioned; externally peer reviewed.